There are two main histological groups of testicular germ cell tumours, which may have different risk factors. Some authors have analysed potential risk factors by histological group but few consistent differences have been identified. In this paper we examine risk factors for pure seminoma and other tumours using data from the United Kingdom case control study of testicular cancer. Seven hundred and ninety-four cases were included in the study, each with a matched control; 400 cases had pure seminoma tumours, and 394 had other testicular tumours. The risk of seminoma associated with undescended testis was slightly higher than that for other tumours (odds ratio of 5.3 compared with 3.0). When split at the median age at diagnosis, this difference was greater in men aged 32 and over (odds ratio of 11.9 compared with 5.1) than in the younger men (3.0 compared with 2.5). Risks associated with testicular or groin injuries were higher in the non-seminoma group, as was the risk for a history of sexually transmitted disease. The protective effect of a late puberty was more marked for tumours of other histologies. Some differences were also detected for participation in sports. Whilst some of the differences detected may have arisen by chance, the stronger association between undescended testis and pure seminoma has been identified by a number of other studies and may reflect a genuine difference in aetiology.